If not i will just use my two old ass 150Gb IDE-hard drives with IDE to USB connectors. It's not gonna look pretty though.
So. Yesterday when DuckTales couldn't fit into Wii U basic fridge space, i finally did it.
It looks like this:

Luckily i managed to hide all that frankenstein thing behind my TV.
First i tried to connect that ancient hard drive to my computer to check if it holds some data i might want to recover from, but it acted up or something and was constantly reconnecting.
Then i gave up on that, and just connected it to Wii U and formatted the whole thing.
It works great so far. Despite that it says it's connected through USB 1 (!), it works at about the same speed as internal memory (Trine 2 loading times are definitely the same).
I had to mess around with saves a bit though, apparently you HAVE to place save and the game data on the same drive, and i honestly would prefer if all my saves went to internal memory, just for safety, in case my old hard drive will die on me.
When i turn my Wii U on, i can hear engine in the hard drive stopping and heads parking, so i think i will be just leaving hard drive power on 24/7, because drive's not working when Wii U is off and because i'm too lazy to mess around with these cables each time.
When i turn Wii U on, it takes some time for hard drive to boot, so Wii U actually boots up faster. Then it shows me some message that i should (re-)connect hard drives only when console is turned off and reboots again. By that time hard drive is fully operational and everything loads up as usual.